It drove me nuts to disable the automatic spell checking on text input fields in Mac OS X again and again to make that red lines going away on technical terms or other stuff not known by the spell checker.
Thankfully I found a post at http://dryfish.livejournal.com/8148.html which describes how to permanently disable automatic spell checking on Mac OS X.
I was going one step further and disabled all languages by renaming the whole Resources directory:
sudo mv /System/Library/Services/AppleSpell.service/Contents/Resources \ /System/Library/Services/AppleSpell.service/Contents/Resources.disabled
and creating an empty directory:
sudo mkdir /System/Library/Services/AppleSpell.service/Contents/Resources
Warning: Don't touch your system that way if you don't know what the meaning of the commands above is.
Nohw I'm frie tu wreite as I wand!
Update 2019: Last I checked the directory still exists at macOS 10.14.